Safety mechanism for materials handling system

ABSTRACT

A safety lock comprises a locking member ( 26, 28 ) which engages a support. An inner lock shaft ( 30   a ) actuates the locking member, the inner lock shaft having a first coupling face ( 35   a ) at one end thereof. An outer lock shaft ( 30   b ) has a second coupling face ( 35   b ) which engages the first coupling face when the inner lock shaft and the outer lock shaft are in respective engagement positions relative to each other. A coupling structure ( 18 ) moves to prevent engagement of said first coupling face with said second coupling face.

This application is a U.S. National Phase Application ofPCT/US2004/004936 filed on Feb. 18, 2004, which claims priority based onU.S. Provisional Patent Application 60/448,382 filed on Feb. 18, 2003.

FIELD OF THE INVENTION

The present invention relates, in general, to supporting a load and, inparticular, to a safety mechanism used with a load. More specifically, asafety mechanism is disclosed which prevents sudden motion of acounterbalanced main arm when the main arm ceases to be in acounterbalanced state.

BACKGROUND OF THE INVENTION

In the manufacture of integrated circuits (ICs) and other electronicdevices, testing with automatic test equipment (ATE) is performed at oneor more stages of the overall process. A special handling apparatus isused to place the device under test (“DUT”) into position for testing.In some cases, the special handling apparatus may also bring the DUT tothe proper temperature and/or maintain it at the proper temperature tobe tested. The special handling apparatus is of various types including“probers” for testing unpackaged devices on a wafer and “devicehandlers” for testing packaged parts; herein, “peripheral” or“peripherals” will be used to refer to all types of such apparatus. Theelectronic testing itself is provided by a large and expensive ATEsystem. The DUT requires precision, high-speed signals for effectivetesting; accordingly, the “test electronics” within the ATE, which areused to test the DUT, are typically located in a test head, which mustbe positioned as close as possible to the DUT. The test head isextremely heavy; the size and weight of test heads have grown over theyears from a few hundred pounds to as much as three to four thousandpounds.

In order to use a test head to test integrated circuits, the test headis typically “docked” to a peripheral. When docked, the test head mustbe located as close as possible to the peripheral's test site in orderto minimize signal degradation. A test head positioning system may beused to position the test head with respect to the peripheral and may bedesigned to facilitate flexible docking and undocking of a test headwith a variety of peripherals. A test head positioning system may alsobe referred to as a test head positioner or test head manipulator. Testhead positioning systems have been described in numerous patents.

In the ordinary operation of a test head positioning system, acounterbalanced arm brings and holds the test head into a desiredposition. Once the test head is in the desired position, the arm may belocked in place. With such systems, if the balance condition is lostwhile the arm is locked in place and this fact is unknown to theoperator of the positioning system, upon release of the lockingmechanism moveable parts of the positioning system will move in a rapidand uncontrolled manner.

The risk of imbalance normally comes in the installing, removing orchanging of test heads. If the positioner system is locked and then oneremoves the test head from the arm but forgets to remove the source ofthe counterbalance (e.g. weights) before unlocking the main arm, thenthe main arm would fly up. Conversely, if the positioner system islocked and one removes the weights without removing the test head beforethe lock is released, then the test head falls.

If one adds or removes too many weights prior to unlocking the arm, thenthere is an imbalance which can cause the test head to fly upwards ordownwards after the lock is released.

Finally, while breakage of the cable that couples the arm to thecounterbalanced system is unlikely, such breakage is not impossible.

U.S. Pat. No. 4,715,574 provides a safety lock system for a materialshandling system such as a test head positioning system. That safety locksystem operates in a manner so that if a balance condition is lost whilethe system is locked, the system cannot be unlocked. More specifically,that system has a safety lock that moves with the main arm. A handle isused for locking the main arm in place. The safety lock preventsrotation of the handle upon a preselected movement of the arm caused bya loss of the balanced condition. If the handle cannot be rotated, thenthe lock cannot be released.

SUMMARY OF THE INVENTION

A safety lock comprises a locking member which engages a support. Aninner lock shaft actuates the locking member, the inner lock shafthaving a first coupling face at one end thereof. An outer lock shaft hasa second coupling face which engages the first coupling face when theinner lock shaft and the outer lock shaft are in respective engagementpositions relative to each other. A coupling structure moves to preventengagement of said first coupling face with said second coupling face.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a first exemplary embodiment of thepresent invention.

FIG. 2 is a top section view of a first exemplary embodiment of thepresent invention.

FIG. 3 is a perspective view of an in-balance condition of a lockcollar, a friction block, and a coupling box when the unit beingpositioned is held in a substantially weightless condition by thecounterbalances.

FIG. 4 is a perspective view of an out-of-balance condition of the lockblock, the panel block, and the coupling box when the unit beingpositioned is in an unbalanced condition.

FIG. 5A is a vertical section taken along line 5A-5A of FIG. 3.

FIG. 5B is a vertical section taken along line 5B-5B of FIG. 4.

FIG. 6A is a vertical section taken along line 6A-6A of FIG. 3.

FIG. 6B is a vertical section taken along line 6B-6B of FIG. 4.

FIG. 7A is a side view of the coupling box.

FIG. 7B is a vertical section taken along line 7B-7B of FIG. 3.

FIG. 7C is a vertical section taken along line 7C-7C of FIG. 4.

FIG. 8 is a first perspective view of a second embodiment of theinvention showing the load carrying unit.

FIG. 9 is a second perspective view of a second embodiment of theinvention without the load carrying unit.

FIG. 10 is a top section view of FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

Referring again to the safety lock disclosed in U.S. Pat. No. 4,715,574,upon a preselected movement of the arm caused by a loss ofcounterbalance to the arm, the safety lock prevented rotation of thehandle that released the lock. It was known to the inventors, however,that certain users of the safety lock forgot its purpose. Thus, whenthose users couldn't turn the handle by hand, they would force thehandle, thus breaking internal teeth which were preventing the lock frombeing released.

Accordingly, the present invention relates to a new and improvedmaterials handling system. Furthermore, a safety mechanism is disclosedfor a materials handling system for a load such as a test head (or otherelectronic testing device). Thus, if a balanced condition is lost whilethe system is locked, the system cannot be unlocked and the load cannotbe moved.

Referring to FIGS. 1 and 2, a first embodiment of a materials handlingsystem, constructed in accordance with the present invention, includes asupport, identified generally by reference numeral 10, which includes avertical support shaft 12. Support 10 may also include a column 14, inthe form of an H-shaped beam (not shown), which extends upward from abase plate (not shown). Additional details about support 10, theH-shaped beam, and the base plate can be obtained by reference to U.S.Pat. No. 4,527,942 and U.S. Pat. No. 4,715,574. Load carrying unit 16 isadapted to receive a load which is to be positioned at a desired heightalong shaft 12. Load carrying unit 16 includes an I-beam (not shown)which is mounted for movement along shaft 12 by means of a pair ofbearing blocks (not shown). Additional details about load carrying unit16 and the manner in which the load carrying unit receives a load forpositioning along shaft 12 can be obtained by reference to U.S. Pat. No.4,527,942 and U.S. Pat. No. 4,715,574.

The materials handling system illustrated in FIGS. 1 and 2 furtherincludes counterbalancing means coupled to load carrying unit 16 forplacing the load carrying unit and the load in substantially weightlesscondition. Only cable 21 of the counterbalancing means is shown in FIG.1.

A lock collar 24 having a bore 25, through which shaft 12 extends, ismovable along the shaft 12. A locking member system within lock collar24 having two locking members (e.g. wedges) 26 and 28 project throughthe wall of the bore 25 in lock collar 24 and engage shaft 12 to lockthe lock collar 24 against vertical movement along shaft 12. This isaccomplished by a rotatable handle 30 coupled to wedges 26 and 28. Wedge26 has a threaded bore 26 a. Wedge 28 has a bore 28 a that is notthreaded. Wedges 26 and 28 have angled faces 26 b and 28 b,respectively, each of which may be tangential to bore 25 and to shaft12.

Lock collar 24 has a bore 24 a which receives wedges 26 and 28 and theinner lock shaft 30 a of a handle 30. A threaded end 30 f of inner lockshaft 30 a engages the threaded bore 26 a in wedge 26. Wedge 28 ismounted on inner lock shaft 30 a by means of a needle bearing 32 androtates freely about the inner lock shaft. The interplay between thethreads on inner lock shaft 30 a and the threaded bore during rotationof inner lock shaft 30 a actuates movement of wedge 26. Clockwiserotation of inner lock shaft 30 a moves wedge 26 toward wedge 28. Whenwedges 26 and 28 are held tightly together, they lock the lock collar 24to shaft 12. Rotating inner lock shaft 30 a counter clockwise moveswedge 26 away from wedge 28. When wedge 26 is a sufficient distance fromwedge 28, lock collar 24 is no longer locked to shaft 12 allowing thelock collar to move vertically along the shaft. The end of inner lockshaft 30 a away from threads 30 f has a coupling face 35 a withjaw-teeth thereon.

A second part of handle 30 is an outer lock shaft 30 b. One end 30 c ofouter lock shaft 30 b has a handle 40 which may be used to turn outerlock shaft 30 b. The other end of outer lock shaft 30 b has a couplingface 35 b with jaw teeth thereon. When jaw-tooth couplings 35 a and 35 bare mutually engaged, rotation of handle 30 in one direction causeswedges 26 and 28 to engage shaft 12 and to fix the position of lockcollar 24 on shaft 12. Otherwise, rotation of handle 30 in an oppositedirection causes wedges 26 and 28 to become disengaged from shaft 12,thereby permitting lock collar 24 to be moved along the shaft.

When inner lock shaft 30 a and outer lock shaft 30 b are aligned, thejaw teeth on coupling faces 35 a and 35 b may be engaged with eachother. When the two jaw-tooth couplings are engaged, rotation of outerlock shaft 30 b causes inner lock shaft 30 a to similarly rotate, whichactuates wedge 26.

A coupling structure 18 is connected to load carrying unit 16 and tofriction block 34 by means of four bolts which extend from the couplingstructure into the friction block 34. Some of the bolts are shown inFIGS. 1-4. Coupling structure 18 has a first face 18 a and a second face18 b. An elongated bore 18 c is located within face 18 a. There are twoconcentric bores 18 d and 18 e in face 18 b. The diameter of bore 18 dis smaller than the diameter of bore 18 e. The depth of bore 18 e withincoupling structure 18 is less than the depth of bore 18 d. Bore 18 dextends from the inside of bore 18 e until it meets bore 18 c.

Jaw-tooth couplings 35 a and 35 b may be located within bore 18 c ofcoupling structure 18. Bore 18 c is large enough to hold jaw-toothcouplings 35 a and 35 b. Bore 18 d is large enough to allow entry androtation of outer lock shaft 30 b.

Outer lock shaft 30 b is a single piece that is machined into twoseparate sections 30 c and 30 d, the diameter of section 30 d being lessthan the diameter of section 30 c. The junction of sections 30 c and 30d forms a lip. A compression spring 31 is located between the annularwall of bore 18 e within coupling structure 18 and the lip formed by thejunction of sections 30 c and 30 d. Compression spring 31 applies ahorizontal force against outer handle section 30 b to force it away fromface 18 b, thereby tending to force jaw-tooth couplings 35 a and 35 bapart. Therefore, even when inner lock shaft 30 a and outer lock shaft30 b are aligned, jaw-tooth couplings 35 a and 35 b may not be engagedbecause of the outward force exerted by compression spring 31 on outerlock shaft 30 b. Jaw-tooth couplings 35 a and 35 b may be engaged whenan operator forces outer lock shaft 30 b in toward inner lock shaft 30 aagainst the compression force of spring 31.

In normal operation, load carrying unit 16 and the load (not shown)carried by the load carrying unit are counterbalanced to permit easymovement along shaft 12 to position the load at a desired height. Next,lock collar 24 is locked in place along shaft 12 by turning handle 30clockwise to cause wedges 26 and 28 to engage the shaft. When loadcarrying unit 16 and its load are to be repositioned, handle 30 isturned counterclockwise to disengage wedges 26 and 28 from shaft 12.

If the balanced system becomes unbalanced while lock collar 24 is lockedto shaft 12 and this fact is unknown to the individual unlocking thelock collar from the shaft, an undesirable condition exists. A loss inthe counterweights causes load carrying unit 16 and its load to movedownward suddenly, while removal of the load will cause unit 16 to moveupward suddenly.

Accordingly, the present invention includes a prevention mechanism thatprevents jaw-tooth couplings 35 a and 35 b from engaging upon apreselected movement of the load carrying unit relative to lock collar24. The prevention mechanism includes friction block 34 and itsrelationship to coupling structure 18 and outer lock shaft 30 b.Friction block 34 bears against and is secured to the I-beam and to lockcollar 24. As shown in FIG. 5A, friction block 34 has a bore 38 throughwhich inner lock shaft 30 a may extend. A series of holes 36 permitbolts such as bolts 19, 20, and 21 to connect friction block 34 tocoupling structure 18.

The coupling of friction block 34 to lock collar 24 and outer lock shaft30 b reacts to the relationship between the weight and counterweight tocause either an alignment or a non-alignment of inner lock shaft 30 aand outer lock shaft 30 b. The two shafts are aligned when the load issubstantially weightless. If the load loses too much of its weightlesscharacteristic, a preselected amount of relative movement between theload and the counterweight causes the inner lock shaft and the outerlock shaft to become misaligned, thereby preventing the outer shaft fromrotating the inner lock shaft 30 a and preventing movement of wedge 26.

Referring to FIG. 5A, the determination of such relative movement isaccomplished, in part, by a plurality of socket head screws 44 havingthe undersides of their heads slip fit against the bottoms of acorresponding number of elongated counterbores 46 provided in that faceof friction block 34. The screws 44 extend through bores 47 in frictionblock 34 into lock collar 24. The opposite face of friction block 34bears against lock collar 24.

Bores 47 are elongated and sized relative to the diameters of screws 44to permit a preselected vertical movement of load carrying unit 16relative to lock collar 24 while the lock collar is locked to shaft 12.The clearance for the screws 44 to move vertically within bores 47 isshown in FIG. 5A. The friction block 34 may slide relative to lockcollar 24 for vertical movement of load carrying unit 16 while the lockcollar is locked to shaft 12.

FIGS. 3, 4, 5A, 5B, 6A, and 6B show how the apparatus may be used todetect relative movement between load carrying unit 16 and lock collar24 when there is an imbalance from the substantially weightlesscondition. Four leaf springs are all attached to the lock collar. Leafsprings 50 and 52 are attached to the top of the lock collar 24 viaplate 66 and fasteners 60 and 62. Leaf springs 54 and 56 are similarlyattached to the bottom of the lock collar. As shown in FIG. 3, when thetops of lock collar 24 and friction block 34 are at substantially thesame level, leaf springs 50 and 52 extend from the top of the lockcollar and bear against the top of friction block 34 during thesubstantially weightless condition of load carrying unit 16. Similarly,leaf springs 54 and 56 extend from the bottom of the lock collar andbear against the bottom of friction block 34 during the substantiallyweightless condition of the load carrying unit. Until the resistance ofleaf springs 50 and 52 against upward movement of friction block 34 isexceeded or until the resistance of leaf springs 54 and 56 againstdownward movement of the friction block 34 is exceeded, the frictionblock 34 remains in place and the teeth of the jaw-tooth couplings maybe engaged.

Referring to FIG. 7A, face 18 a of coupling structure 18 is shown. Holes37 are adapted to receive bolts such as bolts 19, 20, and 21 whichconnect the coupling box to load carrying unit 16, lock collar 24, andfriction block 34. FIG. 3 also shows the teeth of jaw-tooth coupling 35b inside bore 18 e. FIG. 7B shows the side of jaw-tooth coupling 35 athat is coupled to inner lock shaft 30 a. Because the load carrying unitis substantially weightless, jaw-tooth coupling 35 a is aligned withjaw-tooth coupling 35 b, thereby preventing jaw-tooth coupling 35 b frombeing seen in FIG. 7A.

When the load carrying unit is substantially weightless, the relativepositions of other parts of the apparatus are shown in FIGS. 3, 5A and6A. As shown in FIG. 3, bolts 19, 20 and 21 are all substantiallyparallel to inner lock shaft 30 a. Since the inner and outer lock shaftsare aligned, bolts 19, 20 and 21 and also all substantially parallel toouter lock shaft 30 b. As shown in FIG. 5A, screws 44 are centeredwithin bores 47. As shown in FIG. 6A, leaf springs 52 and 56 extend in aflat manner from lock collar 24 to and against the top of friction block34. This orientation of the leaf springs with the lock collar and thefriction block 34 means that the friction block has not moved relativeto the lock collar. Consequently, jaw-tooth couplings 35 a and 35 b arealigned and may be engaged. Because of the alignment of jaw-toothcouplings 35 a and 35 b, handle 30 may be turned to release wedges 26and 28 from shaft 12.

FIGS. 4, 5B, 6B and 7C show the orientation of the elements of theinvention when the load carrying unit is not substantially weightless.Under that circumstance, when the resistance of leaf springs 50 and 52against upward movement of the friction block 34 is exceeded, frictionblock 34 moves upward forcing leaf springs 50 and 52 upward. The effectof the upward movement of coupling structure 18 is shown in FIGS. 7B and7C. A comparison between FIGS. 5A and 5B shows that the relativepositions of screws 44 and bores 46 has changed. In FIG. 5A, screws 44are in the center of bores 47, the tops of the bolts in upper holes 36are substantially level with the tops of upper screws 44, and thebottoms of the bolts in lower holes 36 are below the bottoms of lowerscrews 44. In FIG. 5B, bores 47 have moved upward along with frictionblock 34 resulting in screws 44 being located at the bottom of bores 47.As a result, the tops of the bolts in upper holes 36 are no longersubstantially level with the tops of upper screws 44. Instead, the topsof upper screws 44 are below the tops of the bolts in upper holes 36.Similarly, the bottoms of the bolts in lower holes 36 are nowsubstantially level with the bottoms of lower screws 44.

The effect of the relative movement between lock collar 24 and frictionblock 34 is shown in FIGS. 6B and 7C. Referring to FIG. 6B, whenfriction block 34 moves upward, it causes the four bolts, includingbolts 19 and 20, to also move upward. It will be understood that theother two bolts, such as bolt 21 shown in FIG. 4, will also move upward.Inner lock shaft 30 a does not move relative to lock collar 24 becauseinner lock shaft 30 a is coupled to wedge 28 and no part of wedge 28 isinside friction block 34. Therefore, inner lock shaft 30 a remainsstationary when friction block 34 moves. As shown in FIGS. 5B, 6B and7A, bolts 19 and 20 are coupled to coupling structure 18 via holes 37.As shown in FIG. 6B, upward movement of bolts 19 and 20 causes couplingstructure 18 to also move upward. Because outer lock shaft 30 b islocated tightly within a small bore in coupling structure 18, outer lockshaft 30 b is misaligned relative to inner lock shaft 30 a therebypreventing engagement of jaw-tooth couplings 35 a and 35 b and furtherpreventing rotation of handle 10 from opening wedges 26 and 28. Therelationship between jaw-tooth couplings 35 a and 35 b duringmisalignment of the inner and outer lock shafts is also shown in FIG. 7Cwherein jaw-tooth coupling 35 b is above, and not aligned or engagedwith, jaw-tooth coupling 35 a.

FIGS. 8-10 show a second exemplary embodiment of the invention.Referring to FIGS. 8 and 9, a second embodiment of a materials handlingsystem, constructed in accordance with the present invention, includes asupport identified generally by reference numeral 110, which includes avertical support rail 112. Support 110 may also include a column 114which extends from a base plate 115. Load carrying unit 116 is adaptedto receive a load which is to be positioned at a desired height alongrail 112. Load carrying unit 116 is mounted for movement along rail 112.

The materials handling system illustrated in FIGS. 8-10 further includescounterbalancing means coupled to load carrying unit 116 for placing theload carrying unit and the load in substantially weightless condition.

A lock collar 124 having an opening 125, through which rail 112 extends,is movable along the rail 112. A caliper lock system within lock collar124 having two calipers 126 and 128 project through the wall of theopening 125 in lock collar 124 and engage rail 112 to lock the lockcollar against vertical movement along the shaft. This is accomplishedby a rotatable handle 130 coupled to calipers 126 and 128. Caliper 126has a threaded hole 126 a. Caliper 128 is not threaded.

A threaded end 130 f of inner lock shaft 130 a engages the threads oncaliper 126. Caliper 128 is also mounted on inner shaft 130 a by meansof a needle bearing (not shown) and rotates freely about the inner lockshaft. The interplay between the threads on inner lock shaft 130 a andthe threads on caliper 126 during rotation of inner lock shaft 130 aactuates movement of caliper 126. Clockwise rotation of inner lock shaft130 a moves caliper 126 toward caliper 128. When calipers 126 and 128are placed tightly against rail 112, they lock the lock collar 124 torail 112. Rotating inner lock shaft 130 a counter clockwise movescaliper 126 away from caliper 128. When caliper 126 is a sufficientdistance from caliper 128, lock collar 124 is no longer locked to rail112 allowing the lock collar to move vertically along the rail. The endof inner lock shaft 130 a away from threads 130 f has a coupling face135 a with jaw-teeth thereon.

A friction block 134 bears against and is secured to lock collar 124.

A second part of handle 130 is an outer lock shaft 130 b. One end ofouter lock shaft 130 b has a handle 140 which may be used to turn outerlock shaft 130 b. The diameter of handle 140 is greater than thediameter of outer lock shaft 130 b. The junction of handle 140 and outerlock shaft 130 b forms a lip. A compression spring 131 is locatedbetween face 134 a of friction block 134 and the lip formed by thejunction of handle 140 and outer lock shaft 130 b. The other end ofouter lock shaft 130 b has a coupling face 135 b with jaw teeth thereon.Outer lock shaft 130 b and coupling face 135 b are located within a borein friction block 134. When jaw-tooth couplings 135 a and 135 b aremutually engaged, rotation of handle 130 in one direction causescalipers 126 and 128 to engage rail 112 and to fix the position of lockcollar 124 on rail 112. Otherwise, rotation of handle 130 in an oppositedirection causes calipers 126 and 128 to become disengaged from rail112, thereby permitting lock collar 124 to be moved along the rail.

When inner lock shaft 130 a and outer lock shaft 130 b are aligned, thejaw teeth on coupling faces 135 a and 135 b may be engaged with eachother. When the two jaw-tooth couplings are engaged, rotation of outerlock shaft 130 b causes inner lock shaft 130 a to similarly rotate,which actuates caliper 126.

As described above, jaw-tooth coupling 135 a may be located within lockcollar 124 and jaw-tooth coupling 135 b may be located within frictionblock 134. Compression spring 131 applies a horizontal force againsthandle 140 and, consequently, against outer lock shaft 130 b. This forcepushes outer lock shaft 130 b away from inner lock shaft 130 a, therebytending to push jaw-tooth couplings 135 a and 135 b apart. Therefore,even when inner lock shaft 130 a and outer lock shaft 130 b are aligned,jaw-tooth couplings 135 a and 135 b may not be engaged because of theoutward force exerted by compression spring 131 on outer lock shaft 130b. Jaw-tooth couplings 135 a and 135 b may be engaged when an operatorforces outer lock shaft 130 b in toward inner lock shaft 130 a againstthe compression force of spring 131.

The second exemplary embodiment of the invention prevents jaw-toothcouplings 135 a and 135 b from engaging upon a preselected movement ofthe load carrying unit relative to lock collar 124. The preventionmechanism includes friction block 134 and its relationship to lockcollar 124. The second embodiment shown in FIGS. 8 and 9 have the samemechanisms as shown in FIG. 5A for sensing misalignment between the loadand the counterweight. These mechanisms are shown generally as bores 147in FIGS. 8 and 9.

Leaf springs 150 and 152 are attached to the top of the friction block134 via clamp 166. Two additional leaf springs (not shown) are similarlyattached to the bottom of friction block 134. When the tops of lockcollar 124 and friction block 134 are at substantially the same level,leaf springs 150 and 152 extend from the top of friction block 134 andbear against the top of lock collar 124 during the substantiallyweightless condition of load carrying unit 116. Similarly, the leafsprings (not shown) clamped to the bottom of friction block 134 extendfrom the bottom of the lock collar and bear against the bottom of thefriction block 134 during the substantially weightless condition of theload carrying unit. Until the resistance of leaf springs 150 and 152against upward movement of friction block 134 is exceeded or until theresistance of the leaf springs clamped to the bottom of the frictionblock 134 is exceeded, the friction block 134 remains in place and theteeth of the jaw-tooth couplings may be engaged.

When the load carrying unit is substantially weightless, inner lockshaft 130 a is aligned with outer lock shaft 130 b and jaw-toothcouplings 135 a and 135 b may be engaged. Because of the engagement ofjaw-tooth couplings 135 a and 135 b, handle 130 may be turned to releasecalipers 126 and 128 from rail 112.

When the load carrying unit is not substantially weightless, and whenthe resistance of leaf springs 150 and 152 against upward movement ofthe friction block 134 is exceeded, friction block 134 moves upwardforcing leaf springs 150 and 152 upward. When there is relative movementbetween lock collar 124 and friction block 134, inner lock shaft 130 aremains stationary because it is located within lock collar 124. Becauseouter lock shaft 130 b is located tightly within a small bore infriction block 134, outer lock shaft 130 b is misaligned relative toinner lock shaft 130 a. The misalignment of the two lock shafts preventsengagement of jaw-tooth couplings 135 a and 135 b thereby preventing theouter shaft from rotating the inner shaft and preventing movement ofcaliper 126.

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimsand without departing from the invention.

1. A safety lock comprising: a locking member which engages a support;an inner lock shaft for actuating said locking member, said inner lockshaft having a first coupling face at one end thereof; an outer lockshaft having a second coupling face which is engagable with said firstcoupling face when said outer lock shaft is in an engagement positionrelative to said inner lock shaft; a coupling structure which moves toprevent engagement of said first coupling face with said second couplingface by moving said outer lock shaft away from said engagement positionrelative to said inner lock shaft.
 2. A safety lock according to claim1, wherein said locking member is a wedge or a caliper.
 3. A safety lockaccording to claim 1, further comprising a lock collar in which saidlocking member is situated, wherein actuation of said locking memberprevents movement of said lock collar along said support.
 4. A safetylock according to claim 3, further comprising a coupling box secured tosaid lock collar and moveable thereto, said inner lock shaft held bysaid lock collar, said outer lock shaft held by said coupling box.
 5. Asafety lock according to claim 4, wherein flexible members secure saidcoupling box to said lock collar.
 6. A safety lock according to claim 4,wherein said outer lock shaft is linearly stationary relative to saidcoupling box.
 7. A safety lock according to claim 3, wherein acounterbalanced load is coupled to said lock collar.
 8. A safety lockaccording to claim 7, wherein said coupling box is secured to said lockcollar via a friction block which is rigidly attached to said couplingbox, said load situated between said friction block and said couplingbox.
 9. A safety lock according to claim 7, wherein said load is a testhead.
 10. A safety lock according to claim 1, further comprising aspring loaded handle coupled to said outer lock shaft, said springurging said first coupling face and said second coupling face apart. 11.A method of locking a load, said method comprising the steps of:operating an outer lock shaft and thus causing an inner lock shaft tocause a locking member to engage a support when respective first andsecond coupling faces of said inner lock shaft and said outer lock shaftare positioned for being engagable with each other; and operating saidouter lock shaft without causing said inner lock shaft to disengage saidlocking member from said support so that said load remains locked whensaid respective first and second coupling faces of said inner lock shaftand said outer lock shaft are positioned away from being engagable witheach other.
 12. A method of locking a load according to claim 11,wherein said locking member is a wedge or a caliper which engages saidsupport.
 13. A method of locking a load according to claim 11, whereinsaid locking member is situated in a lock collar, a coupling box issecured to said lock collar and is movable thereto, said inner lockshaft is held by said lock collar, and said outer lock shaft is held bysaid coupling box, said inner lock shaft and said outer lock shaft areaway from engagement with each other when said coupling box movesrelative to said lock collar.
 14. A method of locking a load accordingto claim 13, wherein said coupling box moves relative to said lockcollar when counterbalancing of a load coupled to said coupling boxchanges.
 15. A method of locking a load according to claim 11, wherein aspring urges said first coupling face and said second coupling faceapart.